since my currently 4GB will be going into another system, i am currently thinking of OCing my i7 860. My cooling is now a Corsair H50 with push/pull giving me idle temps at about 25°C.
I think of Ocing only to 3,6Ghz since i prefer lower temps due to summer lanparty etc.
right now i am considering to purchase one of this two:

I would go with the 1866 since it would give you head room to oc the ram if, need/wanted.

You should be fine with temps even oc'ed to 4ghz.. a corsair H50, push/pull set up, should make it to where it's idling at 40-45c and no higher then 75c.. But that is a ruff estimate on it. Due to me living in Florida, I quite understand the worring on chip and summer days..

It's called knowing the limits of the device. The first thing that you may ever kill via ocing is your video card. Your motherboard may die before your chip even has anything wrong with it. If you keep the temps right within the system, keep things cool, your oc's will never hurt. Just gotta Know the limits of what your playing with.. If you don't.. you're going to kill something and it won't be pretty once you see how much your going to put out of your wallet.

Thanks for all the useful replies. Actually i had a Bequiet 750W PSU.
It is going now into another computer (GF uses it, should be enough for my EVGA 470, when my Zotac AMP gets delivered) and i purchased a Corsair HX850W.

I can remember someone who OCed his LGA775 Cpu and forgot to set the PCIe Frequency to 100Mhz. Manufacturer declined to repair his components when he tried to RMA them. So you might allways do enough research in the web before getting started.

Two of my four sticks got grilled when i OCed a Celeron 400 to 600. Only two sticks were capable of running 100Mhz. That "mistake" not that cheap back then.. :/